Central switching systems (referred to below generally as switching systems) typically have trunks connected to them as paths to communicate with other switching systems, telephone lines to communicate with station sets, and central office trunks or lines to communicate with PBXs. The switching systems typically operate to interconnect telephone lines with each other or with trunks to other switching systems or PBXs. The switching systems can, however, operate to provide a service such as Centrextm, whereby the systems provide services to a group of telephone lines as if a PBX were interfacing the group of telephone lines and the switching system (e.g. to provide specialized user services such as speed dialing, etc.)
Where a typical PBX is used, a user must dial the directory number of a called party, and the PBX analyzes either the complete dialed number or the first digit or few digits, then assuming that the call is destined to be completed to a line outside the PBX, seizes an appropriate central office trunk to the switching system. Then, typically using dual tone multifrequency (dtmf) signals, the PBX sends the dialed digits to the switching system. The switching system then completes the call in the usual way as if the call had come from a local line.
The PBX-switching system combination provides a concentration function, wherein the number of trunks interconnecting the two is significantly smaller than the number of calls originating at the PBX (since many of the calls originating at the PBX are destined for completion at the PBX without requiring the switching system). On the other hand, if the switching system is to provide a Centrex function, the number of lines terminating at the switching system must be the same as the number of lines that could require service (otherwise Centrex service could not be provided to all lines).
Where a PBX is used, the amount of time required to communicate dialed digits to a switching system is either excessive, or requires use of extra data lines in addition to the central office trunks. Further, the central switching system in this case cannot be used to provide a Centrex function, since it has no knowledge of which line connected to the PBX requires specialized services.
Some PBXs can communicate with a central switching system and which allow the central switching system to provide Centrex type service, but they suffer from the same deficiencies as noted above.